Rubber treatment



any.

United States Patent RUBBER TREATMENT Kenneth W. Doak, Bloomfield, and Robert A. Gregg,

Passaic, N. J., assignors to United States Rubber Company, New York, N. Y., a corporation of New Jersey No Drawing. Application August 31, 1954, Serial No. 453,444

6 Claims. (Cl. 260-415) This invention relates to improvements in the technique of processing carbon black and rubber mixes prior to vulcanization thereof.

The technique of processing rubber mixes loaded with carbon black, prior to vulcanization thereof, whereby to obtain vulcanizates with improvements in physical and chemical properties, is described in Gerke et al. U. S. Patent 2,118,601. The improved vulcanizates prepared by the technique of Gerke et al. differ from the usual vulcanizates produced by older techniques in that they have relatively (1) lower modulus at low elongation, (2) higher modulus above 300% elongation, (3) higher resistance to abrasion, (4) lower torsional hysteresis, and (5) higher electrical resistivity, and are (6) relatively softer.

The improved vulcanizates of Gerke et al., are obtained by incorporating uniformly in the rubber a relatively large amount of carbon black, for example, at least 25 parts, and in the case of tire treads at least 40 parts, by weight of carbon black per 100 parts by weight of rubber, and subjecting the mixture to a heat treatment at a temperature substantially above 250 F., the preferred temperature being in the range from about 300 F. to 370 F., and masticating the mix during and/or after such heat treatment, or alternately therewith. The duration of the special heat treatment varies with the temperature employed, the higher the temperature the shorter the time, and is governed also by the degree of change desired in the properties of the ultimate vulcanized product which properties are gauged to be compatible with its final use. In general, heat treatments of from 10 to 60 minutes duration are said to be suitable for most purposes, particularly within the preferred temperature range.

The principal object of the present invention is to provide a new chemical promoter for the processing of rubher and carbon black mixtures at relatively high temperatures, as in processes of the general type disclosed in the above-mentioned Gerke et al. patent. Such processing is often referred to as low-hysteresis processing and is usually designedto produce high electrical resistance and low torsional hysteresis, particularly in rubber stocks used for the manufacture of pneumatic tires and more particularly tread stocks. A further object is to bring about a substantial reduction in the time of such processing by the use of the herein disclosed chemical whereby an important increase in the capacity and output of the equipment is obtained with the result that such processing is rendered commercially feasible. Other objects and advantages of our invention will more fully hereinafter appear.

The present invention is based upon our discovery that carbon tetrabromide substantailly. decreases the time and/or lowers the temperature necessary for so-called low-hysteresis processing of rubber and carbon black mixes, that is, the rate of the low-hysteresis processing reaction, at a given temperature, is materially increased.

The process of our invention comprises mixing rubber with a relatively large amount of a rubber-reinforcing carbon black and a relatively small amount of carbon tetrabromide and heating this mixture at a temperature of from 275 F. to a temperature just short of that at which the properties of the rubber would be injured by thermal decomposition to bring about the desired changes in the rubber and carbon black mixture. With the resulting heat-treated mixture there are then incorporated vulcanizing agents, almost invariably including sulfur, and other desired compounding ingredients including conventional accelerators and the like, these being intimately incorporated in any suitable way after which the mixture is shaped and vulcanized in the usual way.

Any carbon black which is capable of reinforcing the rubber can be used in the practice of our invention. We generally employ either furnace black or channel black. It will be understood that the particular type of carbon black will generally be selected with reference to the particular rubber used, this being a matter well within the skill of the art. The amount of carbon black present during the heat treatment should be equal to at least 25 parts per 100 parts by weight of rubber. Preferably the amount of carbon black is equal to at least 40 parts per 100 parts of rubber, the use of such high proportions of carbon black being particularly desirable in the case of tread stocks. The amount of carbon black present during the heat treatment can range as high as 100 parts per 100 parts of rubber.

Our invention can be practiced with any vulcanizable, aliphatic conjugated diolefin hydrocarbon polymer rubher, e. g., Hevea rubber and those synthetic rubbers which are either homopolymers of aliphatic conjugated diolefin hydrocarbons, especially butadiene and isoprene, examples of such homopolymers being synthetic polybutadiene and polyisoprene, or coploymers of such aliphatic conjugated diolefin hydrocarbons with other ethylenically unsaturated copolymerizable monomeric compounds, e. g., copolymers of butadiene with styrene, isobutylene, alphamethylstyrene, acrylonitrile, monovinylpyridines, ethyl acrylate, methyl vinyl ketone, methyl isopropenyl ketone, methyl acrylate, and methyl methacrylate. v Our invention is applicable with Butyl rubber which, as is well known, is a synthetic rubbery copolymer of a major proportion of isobutylene, typically 99.5% thereof, and a minor proportion, typically 10- 0.5%, of an aliphatic conjugated diolefin hydrocarbon especially butadiene or isoprene.

The amount of carbon tetrabromide we prefer to employ in the practice of our invention is from 0.75 to 5 parts per parts of rubber. Amounts materially less than 0.75 part do not give the desired promoting effect and amounts much greater than 5 parts are uneconomical to use.

The carbon tetrabromide should be incorporated with the rubber and carbon black at relatively low temperature, typically not over 250 F., to avoid premature reaction of the carbon tetrabromide with the rubber. Such premature reaction would lessen the eificiency and eifectiveness of our treatment.

In the preferred practice of our invention, the heat, treatment of the mixture of rubber, carbon black and bury mixer, and other factors, extraneous heat may or may not need to be applied to bring the stock temperature within the desiredtemperature rangeand hold it there. In some cases it may be necessary to apply extraneous cooling to keep the stock temperature from rising above the desired level.

The optimum duration of the masticatory heat treatment may vary widely depending upon rnany factors including the temperature of the heat treatment, type of mixer, amount of carbon tetrabromide, etc. In any case it will be considerably shorter, under like. temperature conditions, than the time required when the promoter isomitted. Times of the order of5' to 20 minutes will generally be adequate for the purposes of our invention.

Generally speaking, when butyl rubber is employed in the practice of our invention, the severity of the heat. treatment in terms of temperature or duration or both, will be greater than when, other rubbers are used.

The process of our invention comprises the following essential steps:

1. Mixing thoroughly the rubber and. the carbon black, and then mixing in the carbon tetrabromide at a relatively low temperature at which the carbon tetrabromide does not substantially react with the rubber.

2'. Heat-treating the resulting mixture, either by mastication or statically, at a stock temperature of at least 275 F. ranging upwardly to a temperature just below that at which the rubber would be injured, for a time sufiiciently long to substantially increase the electrical resistivity and substantially decrease the torsional hysteresis of a vulcanizate of the heat-treated mixture.

3. Masticating the mixture, either during or subsequent to the heat treatment or alternately therewith. Where the heat treatment is done statically, this mastication may be performed incident to step 4.

4. Incorporating vulcanizing ingredients, almost invariably including sulfur, in amount and ofa type such asto completely cure the rubber, and any other desired com pounding ingredients. These ingredients are usually in corporated during mastication following the heat treat ment. They are incorporated at a temperature sufiiciently low to preclude vulcanization.

S; Shaping.

6. Vulcanizing the shaped mixture.

The following examplesillustrate the preferred method. of practicing our invention. All parts expressed herein. are-by weight.

Example 1 tetrabromideat a. temperature below 250? F. The-mill temperature is raised to 300 F. and the mixture is masticated for minutes. Thereafter the mill iscooled to 150-200 F;, and. 2" parts ofpinet-ar, 2 parts of'zinc oxide,

one part of antioxidant, one part of accelerator and'2;6 par-ts of sulfur are incorporated; The'mixture is vulcanized in a suitable mold for 45 minutes at 287E As a control, an identical mastcrbatch is pnepamdand sub;

iected to the same manipulative, steps. except, that no.

carbon tetrabromide is. added. The. specific electrical. resistivity and the. torsionalhysteresis aretmeasnred', with the following results:

The practice of the invention has increased the specific electrical resistivity bya factor ofatleast 900,000, and has reduced the torsional hysteresis by about 60%.

Example 2 To a masterbatch of 100 parts of a butadiene-styrene copolymer (GR-S polymerized at 41 F.) and 55 parts of carbon black (high abrasion furnace black known as Philblack O) and 6 parts of hydrocarbon softener is added 3.3 parts of: carbon tetrabromide at about 200 f. This mixture is then masticated for 10 minutes on amill at about. 300 F. The. mill is. cooled to 200 F. andiLS parts of zinc oxide, one part of stearic acid, 2 parts of sulfur, and a suitable accelerator are added. The stock is vulcanized for 30 minutes at 293 F. As a control, an identical masterbatch. i's; subjectedato the same manipulative steps, except that no carbon tetrabromide is used. Specific electrical resistivity and torsional hysteresis are measured, with the following results:

The practice of the invention has increasedthespecific; electrical resistivity by a factor of over 10,000,, andthas reduced the torsional hysteresis by- 37%.

Example-3 To a masterbatch. of 100 parts of a copolymer of isoprene and isobutylene (known commercially as Bntyl. l5) and 50 parts of carbon black. (Spheron 6) is added 1.5 parts of carbon tetrabromide. Thismixture ismasticatedi for 15' minutes at 375" F. in a Banbury mixer. Vulcanizing ingredients (one, part ofstearic acid, ipartsof zinc oxide, 0.5 part; of2mercaptobenzothiamle,.foneV part of tetramethylthiuram disulfide, andv 2, parts. of sulfur) are mixed into the stock at about 200 F. The, stock is vulcanized'for 45 minutes at 293 F. Alcontrol'. stock is subjected: to. all. the previously described manipu: lative steps except that no carbon tetrabromid'e isused. Specific electrical resistivity and torsional hysteresis aremeasured; with the following results:

I Tors Log Re-- Promoter t F sjsnvity K None 64; 6.9: 0.166 Carbon tetrabromlde... 49 13.0, .025

1 'Iors Tors Promoter, ML-4 LogRe- 7 :Log R (212 a sistivity 5&3 Pmmm sistiyitm -ggiii'.

None..... 4a 7.2 0.121,,- N pem1: 0.464.. Carbontetrobromide. 28 131) .051, flarbontetrabromlde n 9.0% 0. 108r Although the specific electrical resistivity is increased by a factor of only 10, the torsional hysteresis is decreased by 34%.

Although we have disclosed our invention with particular emphasis upon the preferred practice wherein the heat treatment is accompanied with mastication, nevertheless our invention can be practiced by carrying out the heat treatment under static conditions. For example, we may form an intimate mixture of the rubber, carbon black and carbon tetrabromide in any suitable manner and then heat this mixture at 275 400 F. without simultaneously masticating it, the heat-treated mixture being subsequently masticated and compounded with conventional compounding and vulcanizing ingredients. The static heat treatment can be conducted by placing slabs of the stock in an oven heated to suitable temperature, or slabs of the stock. can be stacked and allowed to stand for several hours, preferably under relatively non-heat-conductive conditions in order to maintain the stock at the high temperature for as long as reasonably possible. It may be desirable to surround the slabs of hot stock with a heat-insulating blanket of any suitable type in order that the residual heat of the slabs may be retained as long as possible.

Static heat treatment has the advantage of releasing the milling or Banbury equipment from use for carrying out the heat treatment of our invention, and this may be highly desirable under some conditions.

The electrical resistivity values given in the above examples were determined by measuring the resistance of a specimen of known thickness (about 0.1 inch) placed between mercury electrodes, under a potential difierence of 135 volts, using a sensitive galvanometer with an Ayrton shunt. The logarithm (to base of the specific electrical resistivity (in ohm-ems.) is designated Log Resistivity.

The torsional hysteresis figures represent the logarithmic decrement (to base 10) of the observed amplitudes of successive oscillations of a torsion pendulum, measured at 280 F. with an apparatus consisting essentially of a torsion pendulum in which the sample of rubber tested supplies the restoring force when the pendulum is deflected. For further description of this test see Gerke et a1. 2,118,601.

Having thus described our invention, what We claim and desire to protect by Letters Patent is:

1. A process which comprises mixing rubber selected from the group consisting of Hevea rubber, synthetic rubbery homopolymers of aliphatic conjugated diolefin hydrocarbons and synthetic rubbery copolymers of such hydro-- carbons with other ethylenically unsaturated copolymerizable monomeric compounds with a relatively large amount of a rubber-reinforcing carbon black and at least 0.75 part of carbon tetrabromide per 100 parts of said rubber,

heating the mixture at a temperature of at least 275 F. but insufficiently elevated to substantially injure the properties of the rubber, masticating the mixture and incorporating vulcanizing and other desired ingredients, shaping the mixture, and vulcanizing the shaped mixture.

2. A process which comprises mixing rubber selected from the group consisting of Hevea rubber, synthetic rubbery homopolymers of aliphatic conjugated diolefin hydrocarbons and synthetic rubbery copolymers of such hydrocarbons with other ethylenically unsaturated co polymerizable monomeric compounds with a relatively large amount of a rubber-reinforcing carbon black and at least 0.75 part of carbon tetrabromide per parts of said rubber, masticating the mixture at a temperature of at least 275 F. but insufiiciently elevated to substantially injure the properties of the rubber, thereafter incorporating vulcanizing and other desired ingredients, shaping the mixture, and vulcanizing the shaped mixture.

3. A process which comprises mixing natural rubber with a relatively large amount of a rubber-reinforcing carbon black and from 0.75 to 5 parts of carbon tetrabromide per 100 parts of said rubber, masticating the mixture at a temperature of from 275 to 400 F., thereafter incorporating vulcanizing and other desired ingredients, shaping the mass, and vulcanizing the resulting shaped mass.

4. A process which comprises mixing a rubbery butadiene-styrene copolymer with a relatively large amount of a rubber-reinforcing carbon black and from 0.75 to 5 parts of carbon tetrabromide per 100 parts of said copolymer, masticating the mixture at a temperature of from 275 to 400 F., thereafter incorporating vulcanizing and other desired ingredients, shaping the mass and vulcanizing the resulting shaped mass.

5. A process which comprises mixing a rubbery isobutylene-aliphatic conjugated diolefin hydrocarbon copolymer With a relatively large amount of a rubber-reinforcing carbon black and from 0.75 to 5 parts of carbon tetrabromide per 100 parts of said copolymer, masticating the mixture at a temperature of from 275 to 400 F., thereafter incorporating vulcanizing and other desired ingredients, shaping the mass, and vulcanizing the resulting shaped mass.

6. A process which comprises mixing a rubbery butadiene-acrylonitrile copolymer with a relatively large amount of a rubber-reinforcing carbon black and from 0.75 to 5 parts of carbon tetrabromide per 100 parts of said copolymer, masticating the mixture at a temperature of from 275 to 400 F., thereafter incorporating vulcanizing and other desired ingredients, shaping the mass, and vulcanizing the resulting shaped mass.

No references cited. 

1. A PROCESS WHICH COMPRISES MIXING RUBBER SELECTED FROM THE GROUP CONSISTING OF HEVEA RUBBER, SYNTHETIC RUBBERY HOMOPOLYMERS OF ALIPHATIC CONJUGATED DIOLEFIN HYDROCARBONS AND SYNTHETIC RUBBERY COPOLYMERS OF SUCH HYDROCARBONS WITH OTHER ETHYLENICALLY UNSATURATED COPOLYMERIZABLE MONOMERIC COMPOUNDS WITH A RELATIVELY LARGE AMOUNT OF A RUBBER-REINFORCING CARBON BLACK AND AT LEAST 0.75 PART OF CARBON TETRABROMIDE PER 100 PARTS OF SAID RUBBER, HEATING THE MIXTURE AT A TEMPERATURE OF AT LEAST 275* F. TIES OF THE RUBBER, MASATICATING THE MIXTURE AND INCORPORATING VULCANIZING AND OTHER DESIRED INGREDIENTS, SHAPING THE MIXTURE, AND VULCANIZING THE SHAPED MIXTURE. 